The present invention relates generally to a vacuum system for evacuating the space between at least two laminae of a laminated assembly and in particular to a vacuum ring construction which is utilized for de.airing a laminated glass assembly such as an automobile windshield, for example, which is an improvement over the ring construction disclosed in U.S. application Ser. No. 710,287 filed Mar. 11, 1985, entitled VACUUM RING FOR PRODUCING LAMINATED GLASS, now U.S. Pat. No. 4,624,731, dated Nov. 25, 1986.
In the early stages of the automobile industry, single sheets of ordinary glass were employed as windshields. As it became evident that this type of windshield presented a considerable safety hazard, the single sheets of ordinary glass were replaced with single sheets of heat treated or tempered glass. Thereafter, laminated safety glass was developed to further reduce the severity of lacerative injuries. The use of laminated glass assemblies greatly increased, until today, when almost all automotive windshields are constructed of some type of laminated glass.
Typically, a laminated glass assembly of the type utilized in vehicle windshields consists of two sheets of glass bonded together with a thin plastic interlayer, such as a sheet of polyvinyl butyral, or the like. In the event of an impact on the laminated glass sufficient to break it, the plastic interlayers function to bind the glass fragments together, thus reducing the risk of injury to a driver or passenger as a result of flying glass or contact with the windshield.
Recently, it has been found that the addition of a second plastic layer to the inner surface of the windshield further reduces the risk of injury. This second plastic layer has been called an anti-laceration shield. Also, it has been found that a single sheet of glass having an anti-lacerative plastic layer applied to its inner surface can serve as a safety windshield or other type of safety window.
A detailed discussion of laminated windshield assemblies and their production methods can be found in U.S. Pat. Nos. 3,808,077 and 4,242,403, both of which are hereby incorporated by reference.
One of the problems of manufacturing laminated glass assemblies of this type is related to the assembling and bonding techniques used to produce a windshield having high optical qualities, characterized by the lack of defects sometimes called "blow-ins." Once the individual laminae of the laminated assembly have been assembled in a stacked arrangement, the individual laminae are pressed toward one another while the entire assembly is heated to a predetermined temperature to cause the layers of plastic material to soften and bond to the adjacent glass sheets. This is typically done in an autoclave. The autoclave chamber is sealed, heated and pressurized, which causes the desired bonding to occur.
Prior to placing the stacked arrangement of individual laminae in an autoclave, it has been found desirable to "de-air" the assembly by evacuating air spaces which may be present between the individual laminae. One method is to place the assembly in a plastic bag and evacuate the bag. A more current method is to use a vacuum ring of the type disclosed in U.S. Pat. Nos. 2,948,645 and 3,074,466. These patents disclose a flexible vacuum ring which is provided with a channel for receiving the entire peripheral edge of the assembly. The channel of the vacuum ring includes a recessed portion which is maintained in a spaced-apart relationship with the entire peripheral edge of the glass assembly, to define a vacuum passage.
In some instances, it is desirable to maintain the vacuum ring in place when the assembly is within the autoclave, and to apply a vacuum to the ring during at least part of the time the assembly is heated and pressurized. However, one of the problems with that approach is that the softened plastic layers are drawn outwardly, past the marginal edges of the glass sheets and into the recessed portion of the vacuum ring. This tends to reduce the thickness of the plastic layers around the marginal edges of the glass, and produce undesirable optical distortion in those areas.